feat: add workspace MCP tool discovery and proxying for chat (#23680)

Coder's chat (chatd) can now discover and use MCP servers configured in
a workspace's `.mcp.json` file. This brings project-specific tooling
(GitHub, databases, docs servers, etc.) into the chat without any manual
configuration.

## How it works

The workspace agent reads `.mcp.json` from the workspace directory (same
format Claude Code uses), connects to the declared MCP servers —
spawning child processes for stdio servers and connecting over the
network for HTTP/SSE — and caches their tool lists. Two new agent HTTP
endpoints expose this:

- `GET /api/v0/mcp/tools` returns the cached tool list (supports
`?refresh=true`)
- `POST /api/v0/mcp/call-tool` proxies calls to the correct server

On each chat turn, chatd calls `ListMCPTools` through the existing
`AgentConn` tailnet connection, wraps each tool as a
`fantasy.AgentTool`, and adds them to the LLM's tool set alongside
built-in and admin-configured MCP tools. Tool names are prefixed with
the server name (`github__create_issue`) to avoid collisions.

Failed server connections are logged and skipped — they never block the
agent or break the chat. Child stdio processes are terminated on agent
shutdown.

agent/agent.go

@@ -50,6 +50,7 @@ import (
 	"github.com/coder/coder/v2/agent/proto/resourcesmonitor"
 	"github.com/coder/coder/v2/agent/reconnectingpty"
 	"github.com/coder/coder/v2/agent/x/agentdesktop"
+	"github.com/coder/coder/v2/agent/x/agentmcp"
 	"github.com/coder/coder/v2/buildinfo"
 	"github.com/coder/coder/v2/cli/gitauth"
 	"github.com/coder/coder/v2/coderd/database/dbtime"
@@ -311,6 +312,8 @@ type agent struct {
 	gitAPI     *agentgit.API
 	processAPI *agentproc.API
 	desktopAPI *agentdesktop.API
+	mcpManager *agentmcp.Manager
+	mcpAPI     *agentmcp.API
 
 	socketServerEnabled bool
 	socketPath          string
@@ -396,6 +399,8 @@ func (a *agent) init() {
 		a.logger.Named("desktop"), a.execer, a.scriptRunner.ScriptBinDir(),
 	)
 	a.desktopAPI = agentdesktop.NewAPI(a.logger.Named("desktop"), desktop, a.clock)
+	a.mcpManager = agentmcp.NewManager(a.logger.Named("mcp"))
+	a.mcpAPI = agentmcp.NewAPI(a.logger.Named("mcp"), a.mcpManager)
 	a.reconnectingPTYServer = reconnectingpty.NewServer(
 		a.logger.Named("reconnecting-pty"),
 		a.sshServer,
@@ -1348,6 +1353,14 @@ func (a *agent) handleManifest(manifestOK *checkpoint) func(ctx context.Context,
 				}
 				a.metrics.startupScriptSeconds.WithLabelValues(label).Set(dur)
 				a.scriptRunner.StartCron()
+
+				// Connect to workspace MCP servers after the
+				// lifecycle transition to avoid delaying Ready.
+				// This runs inside the tracked goroutine so it
+				// is properly awaited on shutdown.
+				if mcpErr := a.mcpManager.Connect(a.gracefulCtx, manifest.Directory); mcpErr != nil {
+					a.logger.Warn(ctx, "failed to connect to workspace MCP servers", slog.Error(mcpErr))
+				}
 			})
 			if err != nil {
 				return xerrors.Errorf("track conn goroutine: %w", err)
@@ -2070,6 +2083,10 @@ func (a *agent) Close() error {
 		a.logger.Error(a.hardCtx, "desktop API close", slog.Error(err))
 	}
 
+	if err := a.mcpManager.Close(); err != nil {
+		a.logger.Error(a.hardCtx, "mcp manager close", slog.Error(err))
+	}
+
 	if a.boundaryLogProxy != nil {
 		err = a.boundaryLogProxy.Close()
 		if err != nil {